histogram observer: ensure buffer shape consistency (#44956)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/44956
Makes buffer shapes for HistogramObserver have the
same shapes in uninitialized versus initialized states.
This is useful because the detectron2 checkpointer assumes
that these states will stay the same, so it removes the
need for manual hacks around the shapes changing.
Test Plan:
```
python test/test_quantization.py TestObserver.test_histogram_observer_consistent_buffer_shape
```
Imported from OSS
Reviewed By: raghuramank100
Differential Revision: D23785382
fbshipit-source-id: 1a83fd4f39b244b00747c368d5d305a07d877c92
